Biomedical Engineering Reference
In-Depth Information
1.1.1
The Structure of Living Organisms
The structure of the organism is realized by tissues. Malpighi (1628-1694),
physician and biologist, studied subdivisions of the bone, liver, brain, spleen,
kidneys, and skin layers, concluding that even the largest organs are composed
of minute glands [29]. In 1835, before the final cell theory (which regards cells
as the basic unit of life) was developed, Jan Evangelista Purkynˇeobservedsmall
''granules'' while looking at the plant tissue through a microscope [30, 31].
As remarked by Pauling, chemical investigation of the plant viruses has shown
that they consist of the materials called
proteins and nucleic acids
.Molecularweight
of the enzyme urease is 483 000. The viruses (looked at sometimes as giant
molecules) with a molecular weight of the order of magnitude of 10 000 000 may
be described as aggregates of smaller molecules [9]. Viruses vary from simple
helical and icosahedral shapes to more complex structures. Most viruses are about
100 times smaller than an average bacterium. Tobacco mosaic virus (TMV) is
a rod-shaped virus of length 3000
˚
A, diameter 150
˚
A, and molecular weight 50
million. Franklin has shown that the TMV protein is in the form of structural
subunits of molecular weight about 29 000 that are arranged on a helix of pitch
23
˚
A and the axial repeat period 69
˚
A [32, 33].
Many microorganisms, such as molds, bacteria, protozoa, consist of single cells,
cf. Figure 1.3. These cells may just be big enough to be seen with an ordinary
microscope, having diameter around 10 000
˚
A(
=
1000 nm
=
1 µm
=
1
×
10
−
6
m),
or they may be much bigger - as large as a millimeter or more in diameter. For
comparison, atomic diameters range between 1 and 2
˚
A. The cells have a structure,
consisting of a cell wall, a few hundred angstroms in thickness, within which is
enclosed a semifluid material called
cytoplasm
, and other components. Other plants
and animals consist largely of tissues - aggregates of cells, which may be of many
different kinds in one organism. The muscles, blood vessels and lymph vessel
walls, tendons, CTs, nerves, skin, and other parts of the body of a man consist of
cells attached to each other to constitute a well-defined structure. There are also
cells that are not attached to this structure, but float around in the body fluids. Most
numerous among these cells are the red corpuscles of the blood. The red corpuscles
in man are flattened disks, about 7500 nm in diameter and 2000 nm thick. There
are about 5 million red cells per cubic millimeter of blood, and a man contains
about 5 l of blood. Some cells are smaller, like the red cells, and some larger - single
nerve cell may be about 1
m in diameter and 100 cm long - extending from the
toe to the spinal cord. A typical cell size is 10
µ
m, and a typical cell mass is 1 ng.
The total number of cells in the adult human body is about 5
µ
10
14
[9]. Groups
of cells combine and form tissue, which combines to form organs, which work
together to form organ systems. The study of tissues is known as
histology
.
Some foreign cells, often nocive, also can dwell in bone. Osteomyelitis (osteo-
from the Greek word osteon, meaning bone, myelo- meaning marrow, and its
meaning is inflammation) means an infection of the bone or bone marrow. The
infection is often caused by bacteria called
Staphylococcus aureus
, a member of the
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